1. Field of the Invention
This invention relates to the field of machine tools, and more particularly to a draw bar mechanism for operating a collet chuck.
2. Background Art
In an industrial machine tool having a rotating spindle, a cutting tool is generally retained in a collet chuck. A clamping force is commonly applied to the collet by a spring loaded draw bar. Due to design considerations such as bearing size and spindle loads, the spring size is subject to practical limitations. It becomes difficult to supply a sufficient collet clamping force and yet provide a reasonable service life. Therefore, in certain prior art draw bar spindles, the force of the spring is multiplied by means of a wedge ring mechanism. Such a mechanism is disclosed in U.S. Pat. No. 4,347,753. In this and similar devices, a number of steel balls are operatively engaged with bearing surfaces on the spindle, the draw bar and a sliding sleeve, respectively. The bearing surfaces are at relative angles of inclination such that a mechanical advantage is realized as the spring loaded sleeve causes the balls to be wedged between the bearing surfaces of the spindle and draw bar.
In the prior art wedge ring devices, a sleeve forces the balls to wedge between the spindle and the draw bar under the force of the spring, but upon release of the clamping force, the sleeve is no longer in positive contact with the balls. Therefore, in order to release the tool from the collet, a release force must be applied to the draw bar to "pinch" the balls out of position. The magnitude of the force that must be applied to the draw bar to pinch the balls out of position is a function of the relative angles of inclination between the bearing surfaces of the spindle and the draw bar, and can be a limiting factor in the design of the prior art devices.
Accordingly, it is one of the objects of the present invention to provide a wedge ring mechanism in which positive control is maintained over the motion of the balls at all times during both engagement and release of the collet clamping force.